A basic principle of a position-sensing accelerometer is that acceleration to which it is subjected forces an attached mass to move in the direction of its sensitive axis. The mass is attached to a spring or similar element that counteracts the movement of the mass, such that the mass moves until the force from the spring balances the force that the mass experiences due to the acceleration. Measurement of the mass's movement in relation to the body of the transducer permits the acceleration itself to be calculated. Other accelerometers are strain-measuring, and have piezo-electric or piezo-resistive material that senses the stress induced by an inertial mass or the strain induced by a spring supporting the mass.
Applications for accelerometers include antilock braking systems, ride suspension systems, in-flight aircraft monitoring, all of which call for small, inexpensive, and reliable devices. For real-time monitoring, in-line digital processing devices control the accelerometer and interpret its output.
Most accelerometers are analog, in the sense that the measurand is an electrical current that varies with the frequency or amplitude of acceleration. However, some accelerometers are digital, in the sense that the "spring" either makes or breaks an electrical contact in response to acceleration. A series of sensing elements are provided, each having an incrementally higher response threshold and each making an electrical contact when its threshold is reached. A digital accelerometer of this nature is described in an article entitled "Microminiature Ganged Threshold Accelerometers Compatible with Integrated Circuit Technology", IEEE Trans. Elec. Devices, ED-19, No. 1, January 1972.
A limitation of many existing accelerometers is that they are large in size relative to the digital controllers and processors used to interpret the accelerometer's output. Size is an especially important consideration for digital accelerometers, whose resolution depends on how many sensing elements are provided. Also, the processes for making many existing accelerometers are not compatible with those used for digital processor devices, so that hybrid assembly is required to combine them with digital processing circuitry. This makes "intelligent accelerometers" expensive to manufacture.
Attempts to reduce the size of accelerometers have led to the manufacture of micro-accelerometers, which may be manufactured using silicon fabrication techniques. These devices typically have masses and springs sculpted from a piece of silicon that exploit the mechanical, as opposed to the electrical, properties of silicon. One type of micro-accelerometer has tiny cantilever beam structures that extend over an etched-out cavity so that they can bend under the force from acceleration. Others use piston-like structures on flexible hinges.
Existing micro-accelerometers have various limitations. The sensing elements often couple motion in more than one direction, and thus do not permit analysis of direction of acceleration. Also, they do not discriminate among different exciting force frequencies. Further, their electrical contacts are susceptible to "stuck on" conditions.
A need exists for an improved micro-accelerometer that is easily integrated with digital processor circuitry, that provides sophisticated analysis of acceleration forces, and that reduces error conditions.